This invention relates to image processing devices for resolution conversion, utilized in image transmission systems, storage apparatus, etc., including scanners, copying machines and facsimile machines.
In image processing systems, it often becomes necessary to convert the resolution of the images. In the case of the facsimile machines, for example, the text and drawing images (without half-tone areas) are first subjected to simple bi-level thresholding. The half-tone images, on the other hand, are subjected to the dithering process. The resolution of the bi-level image that is obtained by these processes is designated by the user. Further, the information is encoded by a coding method such as MMR. When this information is transmitted to another facsimile machine, the resolution of the image must be converted such that the format of the information is adapted to the paper size and the resolution of the type of the machine to which the image is transmitted. Thus, an image scanned in an inch-based Group 4 facsimile machine should be subjected to an appropriate resolution conversion before being sent to a millimeter-based Group 3 machine.
The most commonly used method for effecting the resolution conversion (or the enlargement/reduction) of the images is the nearest neighbor method (NNM). According to this method, the input and the output sampling points are plotted upon the image to be converted, and the signal level at the input sampling point nearest to the current output sampling point is selected as the output signal level.
The nearest neighbor method (NNM) is a simple and efficient method for resolution conversion. However, this method has the defect that thin lines may be dropped. Thus, for obtaining better conversion image quality, various methods of resolution conversion, such as the method of taking a logical sum, the method of dividing into nine partitions, and the rapid projection method, are proposed. For details of such methods, the reference may be had, for example, to H. Morita et al., "RAPID PIXEL DENSITY CONVERSION METHOD BASED ON THE PROJECTION METHOD", Proceedings of The Institute of Image and Electronics Engineers of Japan, Vol. 11, No. 2, pp. 72-83, 1982.
These methods of resolution conversion, however, are primarily aimed at converting the binary image consisting of texts and drawings without half-tone areas. When these resolution conversion methods are applied to quasi-half-tone input image signal, which is obtained, for example, by means of the ordered dither method (ODM) or the error diffusion method, the grey scale gradation may be degraded, or moire patterns may be developed in the converted image. The quality of the converted image thus suffers a conspicuous degradation.